1. Field of the Invention
The present invention relates to vehicle intruder detection and alarm systems incorporating pressure operated switches.
2. Discussion of prior Art
The simplest form of vehicle intruder detection system incorporates switches into the doors in the form of electrical contacts, which are then arranged in a circuit so as to trigger an alarm when a door is opened. Such switches are usually configured so as to close when a door is opened, in a similar mechanism to that employed for operating courtesy lights, thereby completing a circuit from vehicle battery to earth to trigger an alarm, and may be additionally located on the bonnet and boot. Although the bonnet and boot switches are made to operate instantaneously, the door switches usually incorporate a few seconds delay. Such a system offers only limited protection against intruders entering a vehicle, and less against theft from a vehicle, in that it detects only an open door and is not triggered when a window is broken or tampered with.
A number of more sophisticated intruder detection systems have been developed which attempt to address the problem of detecting attempted intrusion into a vehicle via the full range of vulnerable sites (door, window, boot, bonnet or similar vulnerable area). A problem common to all such alarm systems has been the difficulty in achieving adequate sensitivity to unauthorized interference with a vehicle without a tendency to suffer from false alarms.
A commonly used system for the detection of unauthorized interference with a vehicle involves the provision of an alarm system including one or more vibration transducers, typically utilizing a piezoelectric material, in combination with a detector and alarm which respond when the transducer is disturbed by a particular level of vibration. These transducers are most usefully located at sites particularly vulnerable to intruder attack, and will generally be located on the doors in the vicinity of the window glass so as to detect disturbance and vibrations in the glass.
Transducers of this type exhibit an analogue response to vibration with a relatively low signal: noise ratio, and as such are particularly difficult to calibrate to adequate sensitivity to ensure that interference with the vehicle is consistently detected without being prone to false alarms. They are susceptible to being triggered by accidental shocks to other parts of the vehicle, such as might be caused by a legitimate passer by, an animal, a child's ball or similar accidental impact with the vehicle, and may even be triggered by the vibrations associated with passing traffic or stormy weather.
More sophisticated vehicle intruder detection alarms may be fitted with ultrasonic sensors, which register changes to the sound signature between transmitter and receiver caused by movement, thereby triggering the alarm. Disadvantageously, the variation in sound signature caused by movement is a relatively weak signal relative to the background noise encountered in practical use, and renders such systems susceptible to problems of calibration. These systems exhibit a tendency to false alarms triggered by noise from passing traffic, and also fail to detect gradual movement, so that, particularly in the absence of conventional door alarms, a vehicle fitted with an ultrasonic system is vulnerable to the intruder who opens a passenger door sufficiently slowly, and objects may then be removed from inside the vehicle without the alarm having been triggered.
The incorporation of devices which detect breaking of a window, such as a switch configured to interact with an intact window in such a way as to be switched to an alternate state on breakage or a strip of conductive material on the glass surface have also been considered. An example of the first type of device is disclosed in DE-A-3 923 608 in which a push button bears against a window when intact and causes an electrical switch to operate when the window is broken. The second principle is well known from strip systems frequently used on large plate glass (eg shop) windows, and relies on the assumption that the material in the strip will sever when the glass breaks, opening the circuit and thereby triggering an alarm. This method has been found to present problems as it cannot be reliably assumed that the window glass will craze uniformly and it is possible that the part of the window to which the sensor material was attached would remain intact, so that triggering of the alarm in the intruder detection system when the window was broken could not consistently be guaranteed. Furthermore, it is generally desirable for an alarm to be triggered by any significant level of interference, even if the window is not immediately broken thereby.
This difficulty can be overcome by applying a conductive coating to the whole window glass surface, so that any break in the glass would affect the resistance of the window, and an alarm could be triggered thereby. Such a system is particularly expensive, both because of the costs arising from application of the coating, and because the only practical quality control test is to break a proportion of the coated windows. It is also impractical to apply to existing vehicles as complete windows would have to be replaced.
Furthermore, all systems which involve a detector located on a window present practical problems with openable windows: the need to attach wires to the windows which do not interfere with the window winding mechanism would necessitate working between the inner and outer skins of the door assembly, making installation complex.